1. Field of the Invention
The present invention relates to a holographic recording and reproducing apparatus and a holographic recording and reproducing method.
2. Description of the Related Art
A holographic memory system is known as a digital information recording system which applies the principle of holography utilizing a recording medium formed of a photorefractive material, i.e. so-called a holographic memory. In this information recording system, the recording information signals are recorded as changes in refractive index on the recording medium of a photorefractive crystal such as lithium niobate single crystals.
There is a conventional holographic recording and reproducing method utilizing the Fourier transform.
FIG. 1 shows a conventional 4f-based holographic recording and reproducing apparatus. A laser light beam 12 emitted from a laser light source 11 is split into a signal light beam 12a and a reference light beam 12b in a beam splitter 13. The signal light beam 12a is expanded in its diameter by a beam expander 14 as collimated light, and then irradiated to a spatial light modulator (hereinafter abbreviated as xe2x80x9cSLMxe2x80x9d) 15 including a dot matrix panel such as a transmission-type TFT liquid crystal display (LCD) panel to which image data to be recorded are provided which are converted by an encoder as electric signals. Thus, the panel forms a bright and dark dot pattern on its plane corresponding to the image data. The signal light beam 12a is optically converted by the dot matrix panel 15 to include data signal components. The signal light beam 12a including dot pattern signal components passes through a Fourier transforming lens 16 which is positioned at a focal distance f apart from the SLM 15. The Fourier transforming lens 16 performs Fourier transformation and then the signal light beam 12a including dot pattern signal components is converged into a recording medium 10. On the other hand, the reference light beam 12b split in the beam splitter 13 is guided to the recording medium 10 by a fixed mirror 17 and a rotary mirror 17a, and intersects an optical path of the signal light beam 12a within the recording medium 10 to form a light interference pattern. The recording medium 10 made of a photorefractive crystal records the spatial intensity modulation represented by the light intensity of the light interference pattern as changes in refractive index corresponding to the data.
In the foregoing manner, the diffraction light from the image data illuminated by a coherent collimated light is focused through the Fourier transforming lens 16 and changed into a distribution on the focal plane, or Fourier plane. The distribution as a result of Fourier transformation is interfered with the coherent reference light to record an interference fringe thereof to the recording medium placed in the vicinity of the focal point. Ending the record of the first page, the rotary mirror 17a is rotated a predetermined amount and parallel moved in position a predetermined amount so that the incident angle of the recording reference light beam 12b on the recording medium 10 is changed to record the second page by the same procedure. In this way, the angle-multiplexed recording is carried out with sequential recording as the above.
In reproducing information, on the other hand, inverse Fourier transformation is carried out to reproduce a dot-pattern image. As shown in FIG. 1 the optical path of the signal light beam 12a is cut off, for example, by the SLM 15 to illuminate only the recording reference light beam 12b to the recording medium 10. In order to make incident the recording reference light beam 12b at the same angle as the recording reference light of upon recording the page to be reproduced, the rotary mirror 17a is changed and controlled in position and angle by the combination of mirror rotation and parallel movement. Reproductive light of the recorded interference pattern appears at an opposite side of the recording medium 10 to the side illuminated by the signal light beam 12a. If the reproduced light is guided to and inverse-Fourier-transformed by an inverse Fourier transforming lens 16a, the dot-pattern signal can be reproduced. Furthermore, if the inverse Fourier transforming lens 16a images the dot-pattern signal on an imaging device or photodetector 20 using a CCD (Charge Coupled Device) or CMOS sensor arranged in the focal point, and reconverted into an electric digital data signal and then sent to a decoder 25, the original data is reproduced.
In this manner, the holographic memory system achieves a great capacity recording of information by multiplexedly recording a great deal of two dimensional data to a certain volume of the recording medium.
The holographic memory system using a photorefractive phenomenon for the recording principle. There is no threshold of recording and deterioration in date within the holographic memory. This matter implies that reference light irradiated to the medium for reproduction gradually erases the recorded hologram which is so called deterioration of reproduction. In the multiplexed recording of information carried out sequentially recording within a certain limited volume of the medium, the first recorded portion suffers from the deterioration of the following recorded portion due to the photorefractive phenomenon having no threshold. Thus the surplus recording is performed in counterbalance to such a deterioration so as to compensate the erased hologram portion. For this, the renewal of recording for so-called scheduling time is required as a so-called refresh operation. The refresh operation should be performed in one lump sum to the entire medium. Thus it is impossible that only one portion of the great quantity of multiplexedly recorded data is partly rewritten by a user as he likes without influencing the other portion in the medium.
In this way, a conventional holographic memory system has any nonvolatile storage and it is difficult to provide a rewritable storage. Further it is hard to partially rewrite an extremely minute portion of the great deal of data stored at a limited one volume region in the recording material because the rewriting of such a minute portion adversely influences the other portion storing the other data.
Thus the present invention has been made in consideration of the above conditions, and an object of the present invention is to provide an apparatus of holographic recording and reproducing and a method therefor in which the user is allowed to rewrite the recorded data in the holographic memory.
According to the present invention, there is provided a holographic recording and reproducing apparatus for recording data on a recording medium and reproducing data from the recording medium, the recording medium being made of a photorefractive material, said apparatus comprising:
a support portion for detachably supporting the recording medium;
a recording-reference-light-beam-supplying-portion for supplying a coherent recording reference light beam having a first wavelength and propagating along a recording optical axis to a major surface of said recording medium;
a signal-light-beam-supplying-portion including a converging lens for converging a coherent signal light beam having the first wavelength which is modulated in accordance with image data, in an optical path into the recording medium such that said signal light beam intersects with the recording reference light beam to produce an optical interference pattern of refractive index with said reference and signal light beams within said recording medium;
a reproducing-reference-light-beam-supplying-portion for supplying into the recording medium a coherent reproducing reference light beam having the first wavelength and propagating in an opposite direction along said recording optical axis of the recording reference light beam to generate a phase conjugate wave from a refractive-index grating of the light interference pattern;
a photo-detecting portion including a receiving lens for receiving said phase conjugate wave and a photo-detector for detecting the dot pattern imaged with said phase conjugate wave to reproduce the image data; and
an image-formation-plane-generating portion for making image-formation planes of said receiving lens and said converging lens coincide with each other to generate a common image-formation plane.
According to one aspect of the present invention of the holographic recording and reproducing apparatus, said image-formation-plane-generating portion includes half mirrors symmetrically disposed with respect to said common image-formation plane in optical paths of said signal-light-beam-supplying-portion and said photo-detecting portion respectively.
According to another aspect of the present invention of the holographic recording and reproducing apparatus, said receiving lens and said converging lens are Fourier transforming lenses symmetrically disposed with respect to said common image-formation plane in optical paths of said signal-light-beam-supplying-portion and said photo-detecting portion respectively.
According to a further aspect of the present invention of the holographic recording and reproducing apparatus, said recording medium has a parallel plate shape.
According to a still further aspect of the present invention, the holographic recording and reproducing apparatus further comprises a gate-light-beam-supplying-portion for irradiating a gate light beam having a second wavelength to a region intersected with said signal light beam and said recording reference light beam within the recording medium in a limited manner to enhance a recording sensitivity of said recording medium.
According to another aspect of the present invention, the holographic recording and reproducing apparatus further comprises a pre-irradiation-beam-supplying-portion for irradiating a pre-irradiation beam having a third wavelength to color said recording medium.
According to the present invention, there is also provided a holographic recording and reproducing method for recording data on a recording medium and reproducing data from the recording medium, the recording medium being made of a photorefractive material, said method comprising the steps of:
making a coherent recording reference light beam having a first wavelength and propagating along a recording optical axis incident onto a major surface of the recording medium;
converging by a converging lens a coherent signal light beam having the first wavelength which is modulated in accordance with image data, in an optical path into the recording medium such that said signal light beam intersects with the recording reference light beam to produce an optical interference pattern of refractive index with said reference and signal light beams within said recording medium;
making a coherent reproducing reference light beam having the first wavelength incident to the recording medium in such a manner that the reproducing reference light beam propagates in an opposite direction along said recording optical axis of the recording reference light beam to generate a phase conjugate wave from a refractive-index grating of the light interference pattern;
providing a receiving lens for receiving said phase conjugate wave and a photo-detector for detecting the dot pattern imaged with said phase conjugate wave to reproduce the image data; and
making image-formation planes of said receiving lens and said converging lens coincide with each other to generate a common image-formation plane.
According to one aspect of the present invention, said holographic recording and reproducing method further comprises the steps of:
producing a phase conjugate wave by irradiating the reproducing reference light beam to a reproduction channel of the refractive-index grating previously recorded in the recording medium in such a manner that said receiving lens reconstructs a real image of the image data on the common image-formation plane of said receiving lens and said converging lens; and
during both the irradiation of the recording reference light beam and the production of the phase conjugate wave, forwarding a reproduced light from the real image to said converging lens to converge it into the recording medium in such a manner that said reproduced light intersects with the recording reference light beam to produce an optical interference pattern of refractive index at a different portion away from said reproduction channel in said recording medium.
According to a further aspect of the present invention, said holographic recording and reproducing method further comprises a step of switching an operation, from the forwarding step of the reproduced light to said converging lens, to the irradiating step of converging by the converging lens a coherent signal light beam having the first wavelength which is modulated in accordance with another image data into the recording medium, simultaneously making the recording reference light beam incident onto the recording medium, to produce an optical interference pattern of refractive index following to said different portion of said reproduction channel.
According to a still further aspect of the present invention, said holographic recording and reproducing method further comprises a step of irradiating a gate light beam having a second wavelength to a region intersected with said signal light beam and said recording reference light beam within the recording medium in a limited manner to enhance a recording sensitivity of said recording medium.
According to another aspect of the present invention, said holographic recording and reproducing method further comprises a step of irradiating a pre-irradiation beam having a third wavelength to color said recording medium.